In a fuel cell system including a polymer electrolyte fuel cell stack to which hydrogen gas (fuel gas) and air (oxidant gas) are supplied to generate electric power, stable power generation is generally realized by supplying to the fuel cell stack an amount of hydrogen gas more than that of hydrogen gas consumed therein. Additionally, the fuel cell system includes a hydrogen gas circulation system whereby, by allowing excess hydrogen gas discharged from a fuel cell stack to be circulated into a hydrogen-gas inlet of the fuel cell stack, the excess hydrogen gas is reused and efficiency of hydrogen gas use is thus enhanced.
In the hydrogen gas circulation system, due to such a cause as nitrogen permeation in the fuel cell stack from an air electrode to a hydrogen electrode, impurities other than hydrogen are gradually accumulated. To cope with this event, in a fuel cell system indicated in Japanese Patent Application Laid-open publication No. 2003-151592, hydrogen gas is circulated by use of a circulation pump while a purge valve disposed in the circulation system is opened at regular intervals. Thereby, a reduction in power generation efficiency is prevented by discharging to the outside of the system the impurities accumulated in the system. It is required to reduce hydrogen concentration of the thus discharged gas before the gas is released to the atmosphere. For that purpose, dilution of the discharged gas is carried out by using air discharged from the fuel cell stack.
With regard to a technology of stopping power generation of a fuel cell stack, Japanese Patent Application Laid-open Publication No. 2003-115317 discloses an operation (hereinafter, referred to as an air-electrode oxygen consumption operation) which allows oxygen in an air electrode to be consumed by, before power generation of a fuel cell stack is stopped, stopping air supply while continuing hydrogen gas supply to the fuel cell stack. Furthermore, Japanese Patent Application Laid-open Publication No. 2004-087244 discloses a technology of exhausting residual gas in a fuel cell stack mounted on a vehicle, the technology utilizing a negative pressure which is generated by utilizing a kinetic energy when the vehicle runs.